Capacitor charger with a modulated current varying with an input voltage and method thereof

ABSTRACT

In a capacitor charger including a transformer having a primary winding connected with an input voltage and a secondary winding for transforming a primary current flowing through the primary winding to a secondary current flowing through the secondary winding, the primary current is adjusted according to a monitoring voltage varying with the input voltage, thereby prolonging the lifetime of the battery that provides the input voltage and improving the power efficiency of the battery.

RELATED APPLICATIONS

This application is a Divisional patent application of co-pendingapplication Ser. No. 11/017,906, filed on 22 Dec. 2004. The entiredisclosure of the prior application Ser. No. 11/017,906, from which anoath or declaration is supplied, is considered a part of the disclosureof the accompanying Divisional application and is hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention is related generally to a capacitor charger, andmore particularly, to a modulation apparatus and method for the chargingcurrent in a capacitor charger varying with the input voltage of thecapacitor charger.

BACKGROUND OF THE INVENTION

Portable apparatus is more and more popular, and therefore capacitorcharger it uses receives more attentions than ever. Furthermore, batteryis typically used for the capacitor charger, since it is the portableapparatus to employ the capacitor charger. Unfortunately, there aredisadvantages to a battery serving as a power source. FIG. 1 is anillustrative diagram of a battery 102 providing a voltage V_(bat) and acurrent I to a load 104. Internally, the battery 102 includes aequivalent resistor R_(S) and a voltage source V_(B), and the voltageV_(bat) it provides equals to the source voltage V_(B) subtracting thevoltage drop across the internal resistor R_(S) asV _(bat) =V _(B) −I×R _(S).  [EQ-1]

FIG. 2 shows an I-V curve of the battery 102 for the voltage V_(bat) andthe current I it supplies, in which the vertical axis represents thesupplied voltage V_(bat), and the horizontal axis represents the loadingcurrent I. From the equation EQ-1, the voltage V_(bat) varies with thecurrent I, as shown by the I-V curve in FIG. 2, the more the current Iis drawn by the load 104, the lower the voltage V_(bat) is provided bythe battery 102. When the voltage V_(bat) becomes lower, the load 104intends to draw more current I from the battery 102, and it will havethe voltage V_(bat) to be further lower, thereby lowering the powerefficiency and shortening the lifetime of the battery 102. When abattery is used to provide the power for a capacitor charger, thebattery cannot provide satisfied efficiency and lifetime.

Therefore, it is desired a modulation apparatus and method for acapacitor charger to improve the power efficiency and prolong thelifetime of the battery the capacitor charger uses.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a capacitor chargerand the modulation method thereof whose charging current varies with theinput voltage, especially supplied by a battery.

Another object of the present invention is to provide a capacitorcharger and the modulation method thereof that could improve the powerefficiency and prolong the lifetime of the battery the capacitor chargeruses.

In a capacitor charger connected with an input voltage, according to thepresent invention, a transformer transforms a primary current flowingthrough its primary winding to a secondary current flowing through itssecondary winding under the control of a current control circuit formodulating the primary current, and a current set circuit is connectedto the current control circuit to adjust the primary current accordingto a monitoring voltage varying with the input voltage.

Since the monitoring voltage is generated depending on the inputvoltage, the current set circuit could monitor the variation of theinput voltage from the monitoring voltage, and therefore, to have thecurrent set circuit to adjust the primary current when the input voltageis varied or lower than a threshold, thereby improving the powerefficiency and prolonging the lifetime of the battery that provides theinput voltage.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsof the present invention taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is an illustrative diagram of a battery providing a voltage and acurrent to a load;

FIG. 2 shows an I-V curve of the battery in FIG. 1 for the voltage andthe current it supplies;

FIG. 3 shows one embodiment of the present invention in an applicationfor a flash lamp module;

FIG. 4A shows the typical primary current I_(P) of the capacitor charger200 when using the alkaline battery and lithium battery listed in Table1;

FIG. 4B shows the two-state switch for the primary current of thecapacitor charger 200 in FIG. 3;

FIG. 5 shows one embodiment for the current control circuit 212 in thecapacitor charger 200 of FIG. 3;

FIG. 6 shows another embodiment of the present invention in anapplication for a flash lamp module;

FIG. 7 shows a further embodiment of the present invention in anapplication for a flash lamp module;

FIG. 8A shows an I-V curve of the capacitor charger 400 in FIG. 7 forits primary current I_(P) and the battery voltage V_(bat); and

FIG. 8B shows the relationship of the on-time period T_(on) for thetransistor 21216 in the capacitor charger 400 and the battery voltageV_(bat).

DETAILED DESCRIPTION OF THE INVENTION

The cut-off voltages of alkaline battery and lithium battery undervarious loading currents are provided in Table 1, where the cut-offvoltage is referred to the minimum of the internal source voltage V_(B)in the battery.

TABLE 1 Set No Load Load 0.5 A Load 1 A Cut-Off Cut-Off Cut-Off BatteryInternal Voltage, Voltage, Voltage, Type Resistance V_(bat) = 2 VV_(bat) = 2 V V_(bat) = 2 V 2 Cell 0.6Ω 2 V 2.3 V 2.6 V Alkaline 1 Cell0.2Ω 3 V 3.1 V 3.2 V Li IonFrom Table 1, it is shown that, when the loading current is higher, thecut-off voltage is also higher. Therefore, lowering the loading currentto follow the lowering of the battery voltage could use the power of thebattery more efficiently.

FIG. 3 shows one embodiment of the present invention in an applicationfor a flash lamp module. A capacitor charger 200 comprises a voltagedivider 202 composed of resistors R₁ and R₂ connected in series betweena battery voltage V_(bat) and ground GND to divide the battery voltageV_(bat) to generate a monitoring voltage V_(D) varying with the batteryvoltage V_(bat) derived from a node 2022 between the resistors R₁ andR₂, a transformer 204 having its primary winding L₁ connected to thebattery voltage V_(bat) and its secondary winding L₂ connected to aflash lamp module 208 through a diode 206, a capacitor 210 shunt to theflash lamp module 208 to be charged and to supply power for the flashlamp module 208, a current control circuit 212 connected to the primarywinding L₁ to control the primary current I_(P), a resistor R_(C1)connected between the current control circuit 212 and ground GND,another resistor R_(C2) connected between the current control circuit212 and a current set circuit 214 through a node 213 between theresistors R_(C1) and R_(C2). The current control circuit 212 and currentset circuit 214 may be integrated in a controller chip for the capacitorcharger 200. A typical power converter circuit may be used in thecurrent control circuit 212 with an additional current set pin toconnect to the node 213. The current set circuit 214 includes acomparator 2142 having two inputs connected with the monitoring voltageV_(D) from the voltage divider 202 and a threshold V_(th), respectively,to generate a comparison signal to switch a transistor 2144, so as toswitch the state of the current set pin.

According to the present invention, the capacitor charger 200 couldimprove the power efficiency and prolong the lifetime of the batterythat provides the input voltage V_(bat) of the capacitor charger 200 bymodulating the primary current I_(P) according to the battery voltageV_(bat). In this embodiment, two-state switch is designed to illustratethe principles of the present invention, i.e., the state of the currentset pin is switched by the transistor 2144 depending on the monitoringvoltage V_(D) varying with the input voltage V_(bat). FIG. 4A shows thetypical primary current I_(P) of the capacitor charger 200 when usingthe alkaline battery and lithium battery listed in Table 1, in whichcurve 216 represents the primary current IP₁ provided by lithiumbattery, and curve 218 represents the primary current IP₂ provided byalkaline battery. FIG. 4B shows the two-state switch for the primarycurrent I_(P) of the capacitor charger 200 in FIG. 3. In particular,under the use of lithium battery, when the battery voltage V_(bat)supplied for the capacitor charger 200 is high enough, the lithiumbattery provides the typical primary current IP₁ as it usually does.While the battery voltage V_(bat) drops lower than a threshold V_(bat)′,the capacitor charger 200 will automatically switches its primarycurrent I_(P) to a lower one, for example IP₂, by switching thetransistor 2144 of the current set circuit 214 in response to themonitoring voltage V_(D) varying with the battery voltage V_(bat). Dueto the switching to the lower loading current, the power efficiency ofthe lithium battery is improved, and the lifetime thereof is prolonged.

FIG. 5 shows one embodiment for the current control circuit 212 in thecapacitor charger 200 of FIG. 3, which includes resistors R₃ and R₄connected in series between the input voltage V_(bat) and ground todivide the input voltage V_(bat) to generate a voltage V_(D2), and anoperational amplifier 21202 having its non-inverting input connectedwith the voltage V_(D2), its inverting input connected to a node 21203,and its output connected to the gate of a transistor 21204 that has itssource connected to the node 21203. Due to the virtual ground betweenthe inputs of the operational amplifier 21202, the node 21203 has thesame voltage V_(D2) thereon, and therefore a current I_(A) is generatedon the drain of the transistor 21204 to serve as the reference currentfor a current mirror 21206 to mirror therefrom to generate a mirrorcurrent I_(B) supplied to a charge/discharge circuit 21208 to furthergenerate a charged voltage V_(C). A comparator 21210 compares thevoltage V_(C) with a reference V_(tf) to generate a comparison signalfor control logics 21212 to generate a control signal through a driver21214 to modulate the on-time period of a transistor 21216 connected inseries to the primary winding L₁ of the transformer 204, therebydetermining the primary current I_(P) flowing through the primarywinding L₁. In this embodiment, the primary current I_(P) also dependson the voltage across the resistor R₅ that could be adjusted by use ofthe equivalent resistance connected to the node 213, and therefore, theprimary current I_(P) could be adjusted by turning on and turning offthe transistor 2144. Theoretically, the combination of the operationalamplifier 21202, the transistor 21204 and the resistor R₅ is identicalto a current source to provide the reference current I_(A) dependent ofthe voltage V_(D2) and the resistance between the node 213 and groundGND. As illustrated by FIG. 3 and FIG. 4B, together with FIG. 5, whenthe battery voltage V_(bat) is at a higher level, the transistor 2144 isturned off, and the equivalent resistance between the node 21203 andground GND is the summation of those of the resistors R₅ and R_(C1).When the battery voltage V_(bat) drops to a lower level to result in themonitoring voltage V_(D) lower than the threshold V_(th), the transistor2144 is turned on, and the resistor R_(C2) is subsequently shunt to theresistor R_(C1) to lower the equivalent resistance between the node21203 and ground GND, which will result in a higher reference currentI_(A) and subsequently shorten the on-time period T_(on) for the switch21216 due to shorter charging time for the voltage V_(C) generated bythe charge/discharge circuit 21208 to reach a sufficient level, so as toswitch the primary current I_(P) from the higher one I_(P1) to the lowerone I_(P2).

Referring to FIGS. 3, 4A and 4B, by exemplarily using alkaline batteryand lithium battery, the primary current I_(P) is switched based on thecurves 216 and 218 shown in FIG. 4A, i.e., the primary current I_(P)will be I_(P1) when using lithium battery, and will be I_(P2) when usingalkaline battery. In the beginning, the capacitor charger 200 is assumedto use lithium battery, and therefore the primary current I_(P) isdetermined to be I_(P1), unless or until the battery voltage V_(bat) islower than the threshold V_(bat)′ so as for the monitoring voltage V_(D)on the output of the voltage divider 202 is lower than the thresholdV_(th), which will have the comparator 2142 to turn on the transistor2144 to switch the primary current I_(P) from I_(P1) to I_(P2), as shownin FIG. 4B. As a result, the power efficiency of the battery isimproved, and the lifetime of the battery is prolonged.

FIG. 6 shows another embodiment of the present invention in anapplication for a flash lamp module. In a capacitor charger 300, thearchitecture of the capacitor charger 200 shown in FIG. 3 is employedlikewise, and the same numerals for those corresponding elements arealso designated hereto, while a voltage selection circuit 302 providesthe monitoring voltage V_(D) to the current set circuit 214 for thecomparator 2142 to compare with the threshold V_(th) to generate acomparison signal to switch the transistor 2144. When the batteryvoltage V_(bat) is lower than a threshold V_(bat)′, the voltageselection circuit 302 switches the monitoring voltage V_(D) from onesetting value to another, and thus the comparator 2142 in the currentset circuit 214 may switch the transistor 2144 from one state toanother, thereby changing the equivalent resistance connected to thenode 213 and subsequently lowering the level of the primary currentI_(P). If the current control circuit 212 and current set circuit 214are integrated in a controller chip, the chip has a battery type pinconnected with the monitoring voltage V_(D), and the voltage selectioncircuit 302 may determine the monitoring voltage V_(D) according to thetype of battery that provides the input voltage V_(bat).

FIG. 7 shows a further embodiment of the present invention in anapplication for a flash lamp module. In a capacitor charger 400, thearchitecture of the capacitor charger 300 shown in FIG. 6 is employedlikewise, and the same numerals for those corresponding elements arealso designated hereto, while a maximum switch on-time setting circuit402 is additionally comprised in the controller chip to set the maximumof the on-time period T_(on,max) to switch the primary current I_(P),for example by the transistor 21216 in the current control circuit 212shown in FIG. 5.

FIG. 8A shows an I-V curve of the capacitor charger 400 for its primarycurrent I_(P) and the battery voltage V_(bat), and FIG. 8B shows therelationship of the on-time period T_(on) for the transistor 21216 inthe capacitor charger 400 and the battery voltage V_(bat). When thebattery voltage V_(bat) gradually drops, the capacitor charger 400 willincrease the on-time period T_(on) for the transistor 21216 to maintainthe primary current I_(P) stable, until the battery voltage V_(bat)touches down to a threshold V_(bat)″, the on-time period T_(on) reachesthe maximum on-time period T_(on,max) set by the maximum switch on-timesetting circuit 402. The variation of the primary current I_(P) could bedetermined by

$\begin{matrix}{{\Delta\; I_{p}} = \frac{V_{bat} \times T_{on}}{L_{1}}} & \left\lbrack {{EQ}\text{-}2} \right\rbrack\end{matrix}$As shown in FIG. 8B, when the battery voltage V_(bat) drops down totouch the threshold V_(bat)″, the on-time period T_(on) reaches themaximum on-time period T_(on,max). After the battery voltage V_(bat) islower than the threshold V_(bat)″, the on-time period T_(on) ismaintained at the maximum on-time period T_(on,max), and from theequation EQ-2, the variation ΔI_(P) of the primary current I_(P) isproportional to the battery voltage V_(bat), due to the constant on-timeperiod T_(on) and inductance L₁. Therefore, when the battery voltageV_(bat) is lower than the threshold V_(bat)″, the primary current I_(P)of the capacitor charger 400 will decrease in follow to the batteryvoltage V_(bat), as shown in FIG. 8A, and the lifetime of the battery isprolonged.

While the present invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

1. A capacitor charger connected with an input voltage, comprising: abattery power source supplying the input voltage, the battery sourcebeing formed by one of a first battery type having a first batteryvoltage and a second battery type having a second battery voltage, thefirst battery voltage being greater than the second battery voltage; atransformer having a primary winding connected with the input voltageand a secondary winding for transforming a primary current flowingthrough the primary winding to a secondary current flowing through thesecondary winding; a current control circuit for controlling a magnitudeof the primary current; and a current set circuit connected to thecurrent control circuit for setting the magnitude of the primary currentresponsive to the battery type forming the battery power source, thecurrent set circuit comparing a monitoring voltage corresponding to theinput voltage with a threshold value to distinguish the first batteryvoltage from the second battery voltage, the magnitude of the primarycurrent being reduced responsive to the second battery voltage beingdetected.